This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/SE99/01662 which has an International filing date of Sep. 23, 1999, which designated the United States of America and was published in English.
The present invention relates to a method and a system for Radio Frequency (RF) control and more exactly to a method and system for tuning the operation frequency and RF electrode system in a cyclotron without the use of any mechanical tuning devices.
Production of radioisotopes normally takes place by means of a suitable particle accelerator, for instance a cyclotron, in which a beam of ions is accelerated. The radioisotopes are formed via nuclear reactions between an incident ion beam and a target medium, which can be a pressurised gas, a liquid or a solid.
The acceleration of ions in a cyclotron is performed via a so-called RF electrode system maintained at high voltage (30-40 kV) oscillating with the same period time as the orbit revolution time of the beam in the accelerating system (or a multiple thereof) as given by the average magnetic field in the cyclotron system and the mass/charge ratio of the accelerated ions.
An RF electrode system is a resonant system consisting of inductive and capacitive elements tuned to a predetermined resonance frequency. Such an RF electrode system is designed for lowest possible power consumption resulting in a design having a quite a narrow bandwidth (equal to a high Q-value). However there is a problem in that the resonance frequency of the RF electrode system will drift over time due to temperature variations and other factors affecting its resonance frequency. Such a drift of the resonance frequency means that the frequency of the applied high voltage oscillation no longer will correspond to the resonance of the RF electrodes in the accelerating system, and such a mismatch leads to a deteriorating function of the acceleration system.
There are found a number of methods for maintaining a frequency match in driven systems. In driven systems the common method is, by means of electrical motors with coupled mechanical tuning elements, to re-tune the RF electrode system to the desired frequency. In some cases the generation of the RF voltage also takes place by a freely oscillating system (i.e., not driven system) which then automatically adopts the oscillation frequency to the resonance of the electrodes. Such a system will normally be rather unstable and requires a higher degree of operator attendance, which is not desirable for a handy system to be used for producing PET isotopes for instance at a hospital facility. Besides, such a mechanical tuning system introduces a number of feeds through the vacuum casing of the accelerator, except for the maintenance problems which may arise regarding the electrical motors used as well as the mechanical tuning system itself.
Accordingly there is a need for a simple system for controlling the acceleration of the ion beam in a cyclotron accelerating system, particular a small system for producing PET isotopes emanating from acceleration of a beam of negative hydrogen ions.
The present invention discloses a method and a system for obtaining a proper resonance of the RF electrodes when using a pre-set or predetermined stable frequency oscillator in a cyclotron accelerator without using mechanical tuning devices. In order to maintain a high RF electrode voltage during operation the RF electrode system resonance is monitored and the frequency of the stable frequency generator is controlled by a feedback system continuously monitoring the matching of the oscillator output frequency and the resonance frequency of the RF electrode system. Necessary small adjustments of the stable oscillator frequency to maintain a maximum matching to the resonance frequency of the RF electrode system are obtained by means of the feedback system to the stable oscillator. The feedback system relies on measured values obtained by a load phase sensor monitoring the output of the final RF power amplifier. A cyclotron control system in turn obtains the set and corrected oscillator frequency value and fine tunes further the magnetic field created in the accelerator device according to the frequency information obtained.
The method according to the present invention is defined by the independent claim 1. Similarly a system incorporating the method is set forth by the independent claim 2 and further embodiments of the system are defined by the dependent claims 3-6.